Designing and Maintaining a Pollution-Resilient …ewh.ieee.org/conf/tdc/McDermott_TD08_PowerQuality.pdf1 Designing and Maintaining a Pollution-Resilient Electric Power System Tom

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Designing and Maintaininga Pollution-Resilient

Electric Power System

Tom McDermott

IEEE/PES T&D ConferenceApril 21-24, 2008 Chicago, IL

4/23/2008 Pollution-Resilent Electric Power System 2

Managing Pollution Issues

• Define the metrics, and measure them• Use simulation to compare different

designs and maintenance strategies– validate with measurements– how to value pollution?

• Implement system changes– again, validate with measurements

2


Harmonic Voltage Distortion

• Pollution Rank #1, PQ Rank #3• Standard: IEEE 519• Metric: THD95; level exceeded 5% of time

– limit at MV is 5.0% total, 3.0% any component


Voltage Flicker

• Pollution Rank #2, PQ Rank #4• Standard: IEEE 1453• Metric: PST; short-term flicker coefficient

– planning limit is 0.9

-100

-50

0

50

100

0.0 0.1 0.2 0.3 0.4 0.5

Step-wise Flicker

Vol

tage

Mag

nitu

de (%

)

Time (s)

3


Transient Overvoltages

• Pollution Rank #3, PQ Rank #5• Standard: IEEE 1159 (plus EPRI reports)• Metric: SATMORI1.7; annual frequency of voltage

transients at least 1.7 per-unit– typical value of 1 per day?


RMS Variations (Sags & Swells)

• Pollution Rank #4, PQ Rank #2• Standard: IEEE 1159 (plus EPRI reports)• Metric: SARFI70; annual frequency of sags to 70%

voltage or below– typical value of 20

(Similar to voltage flicker)

4


Momentary Interruptions

• Pollution Rank #5, PQ Rank #1• Standard: IEEE 1366• Metric: MAIFI; frequency of interruptions up to 1 or 5 min

– typical value of 6 annually

(Sags below 10% voltage; followed by a voltage recovery transient when the fault clears)


Benefits of Source Strength

• Reduce voltage sag magnitudes• Absorb more harmonic current injection• Reduce flicker magnitudes• Note: these are probably what you think of

as “pollution”– No inherent effect on interruptions (aka

reliability) or transients (another type of pollution)

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Source Effect on Sags

1PCCA

S A

ZVZ Z− =

+ 2PCCA

S F A

ZVZ Z Z− =

+ +

Fault on Adjacent Feeder Downstream Fault


0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

10 20 30 40 50 60

Sk [MVA]

Pst*10TDD [%]Pst Lim

Source Effect on Flicker & Harmonics

2

KS F

kVSZ Z

=+

NST f

K

SP cS

=

6


Source Effect Parallel Resonance

0

50

100

150

200

250

3 4 5 6 7 8 9

Parallel Resonance of 5 MVA, 13.8-kV CapacitorV

olta

ge (V

)

Frequency (H pu)

20-MVA Transformer 12-MVA Transformer


How To Increase Source Strength

• Increase conductor sizes• Increase substation transformer capacity• Add a substation transformer• Higher transmission voltage service• Add a transmission line• Express feeders (isolation from other load)• Dedicated transformers (more isolation)

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Benefits of Sectionalizing

• Better interruption indices• More RMS variations (they were

interruptions before)• No inherent impact on harmonics, flicker,

or transients


Effect of SectionalizingPermanent Fault

Temporary Fault

SARFI SAIFI

SARFI SARFI SAIFI

MAIFI

MAIFI

SARFI

SARFISARFI

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Impact of Distributed Resources

• The effect is mixed, and benefits may accrue primarily to the DG owner

• Good Impacts:– Higher source strength? Depends on interconnection– Backup source may reduce long-term interruption

indices• Bad Impacts:

– Higher ground fault voltages on healthy phases– May produce more flicker or harmonics


DG Effect on Overcurrent Protection

• No Fuse Saving

• Loss of Sensitivity

• Sympathetic Tripping

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DG Impact on Reclosing

• DG has to disconnect early in the reclose interval, may lengthen to 2 – 5 seconds

• Dead time changes from “instantaneous” to “momentary” category of IEEE 1159

Reclose Interval

DG Must Disconnect Here


Reducing the Event Rate

• Helps both interruptions and RMS variations

• Upgraded lightning protection– Arresters– Overhead shield wires, with better grounding– More insulation

• Vegetation management

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Other Measures

• Capacitor switching surge reduction– Synchronous or zero-voltage control– Preinsertion resistors

• Power electronic devices to reduce flicker or harmonics (STATCOM, DVAR, etc.)

• Passive harmonic filters• PWM converters don’t inject low-order

harmonic currents


Capacitor Switches

45.01.897Enhanced-Duty Ind.28.02.088Standard Inductor

0.01.56520-Ω Resistor94.52.010None

% >= 1.7 p.u.Peak [p.u.]Mitigation

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No Mitigation

-2

-1

0

1

2

0 10 20 30 40 50

gV

olta

ge (V

pu)

Time (ms)


Pre-insertion Resistor

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

0 10 20 30 40 50

Vol

tage

(V p

u)

Time (ms)

12


Pre-insertion Inductor

-2

-1

0

1

2

80 100 120 140 160 180 200 220 240

yV

olta

ge (V

pu)

Time (ms)


Why Do We Need PQ Simulation?

• Measurements take a long time• Measurements are after the fact• Simulation can be a virtual laboratory

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Stochastic Fault Application

• Rate (0.1 / mile / yr)• Other Variables:

– Phases Involved– Permanent? (20%)– Resistance? (5 Ω)

• Each type makes a weighted contribution to MAIFI and SARFI


Fault Analysis Flowchart

Coil Pickup

Contact

Time dial

Trip and Reset Times

0.1

1

10

100

0.01 0.1 1 10 100

Multiples of Pickup (M)

Seco

nds

14


Stochastic Load Variations

0

100

200

300

400

500

600

0 500 1000 1500 2000 2500

Time [s]

Pow

er

0

50

100

150

200

250

190

230

270

310

350

390

430

470

510

550

590

Power [MW]

Freq

uenc

y

0%10%20%30%40%50%60%70%80%90%100%

Peak = 579.6 MW

2% Level = 563.6 MW

5% Level = 533.6 MW


How Accurate Can This Be?• Overcurrent device models are very good

– Utilities used these for decades– They have been essential to protect life and property

• Voltage and current calculations are very good– Utilities used these for decades– They have been essential for equipment specification

• The greatest uncertainty lies in the number, types, and locations of faults, or the variation of harmonic and flicker injections– Utilities do gather some statistics– Benchmark over time with PQ measurements

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Conclusion

• You can’t manage something unless you measure it

• Software tools need to produce figure-of-merit outputs (SARFI, MAIFI, PST, THD95)

• How to assign value to the indices?• EPRI software tools:

– PQ Planner (not used much)– DSS (going open source!!)

Documents

Designing and Maintaining a Pollution-Resilient …ewh.ieee.org/conf/tdc/McDermott_TD08_PowerQuality.pdf1 Designing and Maintaining a Pollution-Resilient Electric Power System Tom